1. Field of the Invention
The present invention relates generally to hard copy apparatus, more specifically to an ink-jet printer employing a heated, planar platen, and particularly to non-warping heated platen assemblies.
2. Description of the Related Art
A variety of hard copy printing technologiesxe2x80x94for example impact, thermal, laser, ink-jetxe2x80x94are commercially available. In order to describe the present invention, exemplary embodiments in the form of ink-jet printers are depicted. No limitation on the scope of the invention is intended by the use of such exemplary embodiments nor should any be implied therefrom. The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIGS. 1A and 1B depict an ink-jet hard copy apparatus in which the present invention is useful; in this exemplary embodiment, an engine for computer printer 101 employing a media vacuum transport is illustrated. In general, the carriage scanning axis is designated the x-axis, the print media transport axis is designated the y-axis, and the pen firing direction onto the media is designated the z-axis. Operation is administrated by an electronic controller (not shown; usually a microprocessor or application specific integrated circuit (xe2x80x9cASICxe2x80x9d) printed circuit board). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions using such a controller.
Paper sheets 22 from an input supply (not shown) are sequentially captured and fed by a vacuum belt mechanism to an internal printing station, or xe2x80x9cprint(ing) zone,xe2x80x9d 28. A thin, endless-loop belt 26 is mounted tightly between belt drive rollers 62, 64. Drive roller 62 is coupled to a stepper device (not shown ) for accurately positioning the sheet in the y-axis with respect to the pen 20. A vacuum box 40, coupled by an appropriate conduit 48 to a vacuum source 50 (FIG. 1B only) has a platen 42 having a plurality of vacuum ports 44 (FIG. 1B only) therethrough. The belt 26 is generally porous, allowing a vacuum flow to pull through the belt via the ports 44. The paper sheet 22 is captured in an upstream (with respect to the pen 20 and associated print zone 28) support zone 55 by the vacuum force exerted thereon as the sheet is received from the input supply and its associated pick mechanisms (not shown). In another upstream, pre-print zone 51, the sheet can be engaged by a controlled pinch roller 53 device. In the print zone 28, one or more ink-jet pens 20, mounted on an encoder controlled scanning carriage (not shown), scan the adjacently positioned paper sheet 22 and graphical images or alphanumeric text are created. Each pen 20 has one or more printhead mechanisms (not seen in these views) for xe2x80x9cjettingxe2x80x9d minute droplets of ink to form dots on the adjacently positioned sheet 22 of print media. Each minute droplet is directed at an artificially imposed row and column grid on the print media known as a picture element (xe2x80x9cpixelxe2x80x9d) using digital dot matrix manipulation to form alphanumeric characters or graphical images. Once a printed page is completed, the print medium is ejected from the belt 26.
For ink-jet printing, it is desirable to maintain a relatively minute, close tolerance, printhead-to-media spacing (z-axis) in order to maximize the accuracy of ink drop placement for optimized print quality. One factor for design optimization is platen flatness. In the state of the art, it is desirable to have a printhead-to-media spacing of less than about one millimeter (xe2x80x9cmmxe2x80x9d). If the platen 42 (or belt 26 riding across the surface thereof) is too close to the printheads at any region of the printing zone 28 or immediately adjacent thereto where pen-to-paper might interfere, smudging of wet ink or damaging pen-media crashes can occur.
To improve ink-jet apparatus performance (ink-media interaction, dry time, print quality, throughput, and the like as would be known to practitioners of the art), it is often advantageous to heat the platen 42. FIG. 2 is an exemplary embodiment of a vacuum belt subsystem 200, including a specific embodiment of a heated platen 42 in accordance with the present invention. A transport portion, or region, 66 of the belt 26 slides over a support surface 52 of the vacuum platen 42, having ports 44 arranged for communicating vacuum pressure to the surface 52. Paper sheets 22 are sequentially directed onto the transport portion 66 by known manner paper supply pick and feed mechanisms (not shown). Conductive heating of the belt 26 is accomplished by the use of one or more heaters 70 that are about 1-millimeter below the platen support surface 52, in this embodiment, fabricated of a ceramic material for conducting the applied heat. The heaters 70 are comprised of an array of printed, linear, resistive heating elements 72. The individual heating elements 72 extend between the rows of vacuum ports 44 that are defined on the support surface 52 of the platen 42. At the edges of the support surface 52, the individual elements 72 are joined (as at reference numeral 74) and the termini of the heaters are enlarged into two contact pads 76 for connecting to a known manner source of electrical potential. The heaters 70 are arranged so that one heater resides on the central portion of the platen 42 immediately in the print zone 28. There are also two heaters 70 in the platen 42 entry region 130, referred to as xe2x80x9centry region heaters,xe2x80x9d viz. a pre-printing operations region. Similarly, two xe2x80x9cexit region heatersxe2x80x9d are provided at the exit region 132 of the platen, viz. A post-printing operations region. Further details of this specific embodiment are described in CONDUCTIVE HEATING OF PRINT MEDIA is described by common inventor Wotton et al., in U.S. patent application Ser. No. 09/412,842, filed Oct. 5, 1999 (assigned to the common assignee herein); however, details other than those incorporated herein are not required in order to understand the present invention.
Under normal operating conditions, the platen 42 may experience temperatures in the approximate exemplary range of zero to 150xc2x0 Centigrade (it will be recognized to those skilled in the art that the actual range will be dependent upon the specific implementation). Such temperature excursions, temperature transients, and cross-platen gradients can cause a platen 42 to warp.
Previous solutions include employing long warm-up time, the use of high cost materials, or providing high power controls (e.g., using 220 volt circuits), and the like to resolve the problems. However, long cool-down times may still need to be employed to ensure flatness is kept within predetermined tolerances.
Therefore, there is a need for methods and apparatus that comprise non-warping heated platen.
In its basic aspects, the present invention provides a heated platen apparatus, having a media transport surface, including: a planar heater, forming said surface and having a predetermined thickness xe2x80x9ctxe2x80x9d; a planar base, having a predetermined thickness xe2x80x9cT,xe2x80x9d substantially greater than xe2x80x9ct,xe2x80x9d and having a low coefficient of thermal expansion; and an attachment conjoining said heater and said base, wherein the attachment provides a high thermal resistance and said surface remains planar regardless of temperature changes of said heater.
In another aspect, the present invention provides a hard copy apparatus, having a means for transporting media through a printing zone, including: a heated, planar, media platen located at least partially within said printing zone, having a planar platen member having a media heating surface, a rigid, planar base, and an attachment for coupling the platen member to the planar base, wherein the base is thermally conductive and relatively thicker than the platen member such that the base heats-up uniformly and does not warp itself to any effective degree due to varying thermal expansions and contractions of the platen member and thereby maintains planarity of the platen member.
In another aspect, the present invention provides a method for maintaining planarity of a heated platen assembly of a printing apparatus, including the steps of: providing a heated platen, an attaching member, and a rigid base; fabricating an attaching member having operational characteristics of the attaching member to ensure flatness of the heated platen by requiring that only a small thermal mass must be heated before printing can begin; and coupling the platen and the base via the attaching member.
Some advantages of the present invention are:
it provides a flat, heated platen over a large temperature range;
it provides a flat, heated platen despite various temperature gradients across the platen;
it provides a flat, heated platen despite rapid temperature transients, e.g., during warm-up and cool-down cycles;
it allows short warm-up times;
it allows rapid cool-down times;
it allows the use of smaller power supplies; and
it allows the heater assembly and platen base to have different coefficients of thermal expansion.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages, or features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01 (d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the basic nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other aspects, objects, advantages, and features of specific embodiments of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.